A litmus test for extreme environments

TitleVisual and Precise Detection of pH Values under Extreme Acidic and Strong Basic Environments by Cellulose-Based Superior Sensor
AuthorsHaq Nawaz, Weiguo Tian, Jinming Zhang, Ruonan Jia, Tiantian Yang, Jian Yu, and Jun Zhang
JournalAnalytical Chemistry
Year2019

Paper pH tests (or litmus tests) are immensely useful for regular monitoring of things like swimming pools or your favorite pseudoscientific alkaline water. These tests, which are derived from lichens, comprise a mixture of compounds which change color based on the pH, or the concentration of protons (H+ ions), of a solution. An acidic solution (an excess of protons) will turn the paper strip red, while a basic solution (a lack of protons) will turn the strip blue. The gradient in between makes it easy to estimate the pH of a solution simply by looking at it.

However, litmus tests only work within a pH range of 4 to 10, which excludes significant liquids that we may be interested in monitoring, such as stomach acid (pH of 1-3.5), industrial manufacturing of paper and leather, or wastewater treatment. Measuring the pH of these strong acids or bases usually requires an electronic pH meter, which requires electricity and calibration, not to mention the extra time and training needed to perform the test.

Researchers at the Chinese Academy of Sciences have developed a pH test that expands beyond the range of traditional litmus tests, and is able to discriminate pH values of 1-2 and 11-14 using a synthetic compound they call Phen-MDI-CA. This new pH test is designed to indicate pH with both the naked eye and fluorescence. Like many dyes, Phen-MDI-CA (Figure 1) contains a conjugated bond system, which easily absorbs light. The functional groups on this molecule gain protons under strong acidic conditions (when there are many extra protons in solution) and lose protons in strong basic solutions, which changes the way the molecule absorbs light. These chemical changes ultimately change the apparent color of the compound.

Figure 1. The structure of Phen-MDI-CA. The name is an abbreviation of 1,10-phenanthroline-5-amine (Phen), 4,4′-methylene diphenyl diisocyanate (MDI), and cellulose acetate (CA).
Adapted with permission from Nawaz, H., et. al. Anal. Chem. 2019, 91, 4, 3085–3092. Copyright 2019 American Chemical Society.

The visual change in color is enough the distinguish between a pH of 1 and 2, where a pH of 2 is clear, and a pH of 1 is a light yellow (Figure 2A). Distinguishing between pH 11-14 is even easier, where 14 is orange, 13 is yellow, 12 is a very light yellow, and 11 is clear (Figure 2B).

Figure 2. pH indication of Phen-MDI-CA in visual and ultraviolet light at (A) high pH and (B) low pH. Adapted with permission from Nawaz, H., et. al. Anal. Chem. 2019, 91, 4, 3085–3092. Copyright 2019 American Chemical Society.

However, if you want a more precise reading, these visual indicators won’t cut it. To create a more precise test, the authors paired Phen-MDI-CA with another fluorescent indicator. When in the same solution, the two molecules provide contrast to give finer detail in the color reading. This strategy is based on ratiometric fluorescence, which is often used to create contrast in fluorescence imaging. The difference in color is dramatic and allows readings at 0.2-0.4 pH intervals (Figure 3).

Figure 3. pH indication with ratiometric references illuminated under visual light (left) and ultraviolet light (middle, right). A, B, and C are all different ratiometric reference molecules. (A) Phen-MDI-CA/CA-PpIX, (B) Phen-MDI-CA/M-G, (C) Phen-MDI-CA/MTPP. Adapted with permission from Nawaz, H., et. al. Anal. Chem. 2019, 91, 4, 3085–3092. Copyright 2019 American Chemical Society.

To finish the experiment, the authors made paper test strips by painting their new compound onto filter paper (Figure 4). The comparison with traditional litmus paper showed the need for this expansion, as there is no difference between the extreme pH values, an no fluorescence at all on the litmus paper.

Figure 4. Paper Phen-MDI-CA pH tests at (A) high pH and (B) low pH. Adapted with permission from Nawaz, H., et. al. Anal. Chem. 2019, 91, 4, 3085–3092. Copyright 2019 American Chemical Society.

People use litmus tests every day to very quickly monitor their swimming pools, their beverages, or their manufactured products. Expanding the capabilities of pH testing can enable a pharmaceutical manufacturer to test their basic reaction conditions or allow a surgeon to quickly test a patient’s stomach acid. We can only imagine the pH-antastic possibilities.

Feature image courtesy of Pixabay.

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